(Start on a new page.)
The graph below shows how the momentum of car A changes with time just before and just after a head-on collision with car B.
Car A has a mass of 1 500 kg, while the mass of car B is 900 kg.
Car B was travelling at a constant velocity of 15 m∙s-1 west before the collision.
Take east as positive and consider the system as isolated.

4.1 What do you understand by the term isolated system as used in physics? (1)
Use the information in the graph to answer the following questions.
4.2 Calculate the:
4.2.1 Magnitude of the velocity of car A just before the collision (3)
4.2.2 Velocity of car B just after the collision (5)
4.2.3 Magnitude of the net average force acting on car A during the collision (4)
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